- the mechanism is about the same: the video stays in the corner
- the term ‘picture in picture’ is already familiar to people, from TVs
...so the term is generalized. Happens a lot. If the feature gets popular and the term sticks around, decades later people might search the web for ‘why is it called PiP when it's not what it does’. We can even imagine that zero videos may be involved by then (though unlikely―there are other terms for that.)
- the mechanism is about the same: the video stays in the corner
- the term ‘picture in picture’ is already familiar to people, from TVs
...so the term is generalized. Happens a lot. If the feature gets popular and the term sticks around, decades later people might search the web for ‘why is it called PiP when it's not what it does’. We can even imagine that zero videos may be involved by then (though unlikely―there are other terms for that.)